Slightly blend stereo audio to support partial deafness

The app to do this for any or all apps or outputs is Rogue Amoeba's SoundSource

This can route any app to any sound device, with EQ… or more importantly, any AU (Apple Audio Unit) Plugin.
macOS comes with a selection of these built-in, but unfortunately not a simple balance/stereo width plugin.

Searching for freeware plugins, I found a discussion at https://community.audirvana.com/t/channel-balance-controls-au/10605/15 recommending some, from which I think Airwindows EveryTrim or Flux StereoTool might be suitable. The trouble with both these is they're just not as straightforward as they could be. They're designed for audio pros. If you struggle, then maybe there's a simpler tool out there… I just can't find one right now.

i found some more listed here - https://www.kvraudio.com/forum/viewtopic.php?t=408421

What may help - depending on whether your hearing loss is broad-spectrum or above a certain frequency range, is to push some 'treble' into one side but not the other.

An earlier app from Rogue Amoeba, Audio Hijack does actually come with the plugins necessary to do this stereo/EQ routing, but it's more expensive & it isn't a 'global' solution, it's more one-app-at-a-time.
I answered an earlier question at the time using Hijack - Mono output from an external DAC and also this one - Split Input Channels Using SoundFlower and AULab
I'm just trying to see if installing the demo of Audio Hijack would put those plugins in the Mac's AU folder so you could use them from SoundSource…
… No. Apparently they're internal to Audio Hijack.